Peak transmission rates in wireless communication systems have steadily increased during the last years. However, peak transmission rates are still limited for example due to path loss, limited spectrum availability and fading.
Transmitter diversity is a highly effective technique for combating fading in wireless communications systems. Several different transmit diversity schemes have been proposed. In Li, Y.; Chuang, J. C.; Sollenberger, N. R.: Transmitter diversity for OFDM systems and its impact on high-rate data wireless networks, IEEE Journal on Selec. Areas, Vol. 17, No. 7, July 1999 the transmit diversity schemes of delay, permutation and space-time coding are exemplarily described. According to the delay approach, a signal is transmitted from a first transmitter antenna and signals transmitted from further transmitter antennas are delayed versions of the signal transmitted from the first transmitter antenna. In the permutation scheme, the modulated signal is transmitted from a first transmitter antenna and permutations of the modulated signal are transmitted from further transmitter antennas. Thus, the signal transmitted from the transmitter antennas can be derived from a matrix composed of data words in the form of the modulated signal and of permutations of the modulated signal. By means of space-time coding a signal is encoded into several data words and each data word is transmitted from a different transmitter antenna. During transmission the data words are spread (i.e., multiplexed) in the time domain by successively transmitting the data symbols of each data word over a single carrier frequency.
Another transmit diversity scheme is described in U.S. Pat. No. 6,088,408. According to this transmit diversity scheme data are coded in the form of matrices and transmitted as individual data blocks. Each data block comprises several data words and each data word contains data symbols derived from an input data signal. During transmission of the data blocks, the individual data words are spread in the time domain. Therefore, the transmit diversity scheme described in U.S. Pat. No. 6,088,408 can be referred to as space-time block coding (STBC). The main features of STBC are that each data symbol is transmitted from each transmit antenna and that the antenna signals of different transmit antennas are orthogonal to each other. Orthogonal STBC data blocks can be designed for an arbitrary number of transmit antennas.
A further transmit diversity scheme for a multi carrier system is space-frequency block coding (SFBC). By means of space-frequency block coding a signal is encoded into individual data blocks comprising several data words and each data word is spread (i.e., multiplexed) in the frequency domain by transmitting the data symbols of each data word on orthogonal frequencies, i.e. orthogonal subcarriers. An exemplary scheme for space-frequency block coding is described in Lee K. F. et al: “A space-frequency transmitter diversity technique for OFDM systems”, Globecom '00-IEEE. Global Telecommunications Conference. Conference record (Cat. No. 00 CH37137), Proceedings of global Telecommunications conference, San Francisco, Calif, USA, 27 Nov. -1 Dec. 2000, pages 1473-1477 vol. 3.
An important feature on a receiving side of a multi carrier transmit diversity system is a characterization of the individual transmit channels which enables an efficient demodulation of the receive signal. Consequently, a channel estimation has to be performed as described in Li, Y.; Chuang, J. C.; Sollenberger, N. R.: Transmitter diversity for OFDM systems and its impact on high-rate data wireless networks, IEEE Journal on Selec. Areas, Vol. 17, No. 7, July 1999 and in U.S. Pat. No. 6,088,408.
Departing from the various channel estimation approaches known in the art there is a need for a more accurate method of estimating channel coefficients in a multi carrier system operating in accordance with a code matrix based transmit diversity scheme in which a data content of the code matrix is multiplexed in the frequency domain. There is also a need for an estimating stage for performing the corresponding estimating method and for a transceiver comprising such an estimating stage .